Conventionally, as a measure against vibrations to be caused when transporting vibration isolation subjects such as a precision apparatus, an electronic apparatus, and an art object, or as a measure against external vibrations such as an earthquake in a case where the vibration isolation subjects are placed in a building, a vibration isolating table has been used for the purpose of isolating those vibration isolation subjects from vibrations of a floor surface.
As such type of vibration isolating table, the vibration isolating table disclosed in JP 2000-240719 A is known. This vibration isolating table is a two-dimensional vibration isolating table including a Y-directional vibration isolating part and an X-directional vibration isolating part which are overlaid one on top of the other relative to the floor surface. The topmost placement table part is freely movable in the X direction and the Y direction relative to the floor surface.
The Y-directional vibration isolating part includes a guiding member fixed to the floor surface, a Y-directional rail that is held so as to be freely movable in the Y direction by the guiding member and is fixed to an intermediate plate, and a plurality of elastic members provided between the intermediate plate and the floor surface so as to hold the intermediate plate at the initial position on the floor surface. Further, the X-directional vibration isolating part includes a guiding member fixed to the intermediate plate of the Y-directional vibration isolating part, an X-directional rail that is held so as to be freely movable in the X direction by the guiding member and is fixed to the placement table part, and a plurality of elastic members provided between the placement table part and the intermediate plate so as to hold the placement table part at the initial position on the intermediate plate.
In the vibration isolating table structured as described above, in a case where vibrations act on the floor surface, the Y-directional vibration isolating part and the X-directional vibration isolating part interact with each other to isolate, from the vibrations of the floor surface, the placement table part on which a vibration isolation subject is placed, and the cycle of the vibrations of the placement table part is set sufficiently long, with the result that the sway of the vibration isolation subject can be suppressed. Further, when the vibrations of the floor surface cease and the vibrations of the placement table part also cease, the placement table part is pulled back to the initial position due to the tensile force of the elastic members.
Further, the conventional vibration isolating table is provided with a damping mechanism for quickly ceasing the vibrations of the placement table part. The damping mechanism is provided both to the Y-directional vibration isolating part and to the X-directional vibration isolating part. The damping mechanism of the Y-directional vibration isolating part includes a frictional member that is opposed to the side surfaces of the Y-directional rail through the intermediation of small gaps and is held so as to be freely rotatable relative to the floor surface, and each of the elastic members is stretched between one end of the frictional member and the intermediate plate. The damping mechanism of the X-directional vibration isolating part has a similar structure, that is, the damping mechanism of the X-directional vibration isolating part includes a frictional member that is opposed to the side surfaces of the X-directional rail through the intermediation of small gaps and is held so as to be freely rotatable relative to the intermediate plate, and each of the elastic members is stretched between one end of the frictional member and the placement table part.
In the damping mechanism structured as described above, when the intermediate plate moves in the Y direction relative to the floor surface or when the placement table part moves in the X direction relative to the intermediate plate, the frictional member rotates due to the tensile force of the elastic members so that the frictional member is brought into press contact with the Y-directional rail or the X-directional rail. As a result, a frictional resistance force is generated. As the distance of movement of the intermediate plate relative to the floor surface becomes larger or as the distance of movement of the placement table part relative to the intermediate plate becomes larger, the elastic members exert a larger tensile force, and hence the frictional resistance force generated between the frictional member and each rail also becomes larger.
In other words, in the vibration isolating table disclosed in JP 2000-240719 A, as the placement table part moves farther away from the initial position, a larger damping force acts on the vibrations of the placement table part.